Hematopoietic stem cells (HSCs) in the mammalian bone marrow (BM) continuously give rise to all blood cell types throughout the entire life. The HSC activity in aging is characterized by multiple abnormalities that prevent the replenishment of peripheral lymphocytes and greatly increase the risk of myeloproliferative disorders and leukemia. To understand precise mechanisms of age-associated HSC impairment, it is necessary to analyze the function of endogenous HSCs in intact aging organisms. This task has been elusive, as standard methods of HSC analysis rely on cell isolation and transfer into irradiated recipients. We have generated a system that allows specific labeling of endogenous HSCs and tracing of their progeny in intact mice. This novel system will be used to characterize the function of endogenous HSCs in aging, using three Specific Aims. In Aim 1, the self- renewal and differentiation of endogenous HSCs will be characterized in old versus young animals. In Aim 2, the clonal dynamics of HSC compartment during aging will be analyzed. In Aim 3, the role of DNA methylation regulators in the function of endogenous HSCs will be studied. Collectively, these studies would directly examine the properties of unmanipulated HSCs during aging, and clarify the mechanisms of aging-associated abnormalities of hematopoiesis.